1. Field of the Invention
The present invention relates to a highly efficient coding apparatus for compressing and encoding a digital picture signal with discrete transform encoding such as discrete cosine transform and, more particularly, to buffering for controlling the amount of data transmission to a value less than a predetermined value.
2. Description of the Prior Art
A picture is divided into blocks, each having a predetermined number of picture elements. In order to suppress the redundancy of a picture signal, transform encoding is performed on a transform axis which matches the characteristic of an original picture signal for every block. Types of known transform encoding include Hadamard transform, cosine transform and Fourier transform. A conventional cosine transform encoding apparatus is shown in FIG. 1.
A sampled discrete picture signal f(j, k) is supplied through an input terminal 51, in FIG. 1, to a cosine transform (DCT) circuit 52 in which a discrete cosine transform, as indicated by the following equation, is performed. Here, the original data are two-dimensional data, that is, f(j, k) where (j, k=0, 1, . . . , n-1), having (n x n) samples/block. ##EQU1##
A coefficient value F(u, v) from the cosine transform circuit 52 is supplied to a block scanning circuit 53, in which coefficient data within a block from a direct current component to a high frequency component are outputted by zigzag scanning. The coefficient data from the block scanning circuit 53 are supplied to a requantization circuit 54, in which the coefficient data are quantized which is controlled by a buffer control circuit 58. The output signal from the requantization circuit 54 is supplied to a sorting circuit 55, in which the coefficient data are sorted in order of the absolute values of the amplitudes and, in which the amplitudes and addresses are differentiated. A differential output signal from the sorting circuit 55 is given coupled to a variable-length encoding circuit 56, which is adapted to convert the signal into a code signal having a predetermined bit number by runlength coding and Haffman coding.
The code signal outputted from the variable-length encoding circuit 56 is supplied to a buffer memory 57 which converts the code signal transmission rate into a rate which does not exceed the rate of a transmission path. More specifically, the rate of data transmission to the input side of the buffer memory 57 is variable, whereas the output rate is nearly constant. The output data from buffer memory 57 are taken out at a terminal 59. A fluctuation of the transmission data rate is detected by the buffer memory 57 and, upon such detection, a detection signal is supplied to buffer control circuit 58.
The buffer control circuit 58 controls the quantization step for the requantization circuit 54, so that, coefficient data to be transmitted become a predetermined data amount by thresholding at the variable-length encoding circuit 56. Thresholding is a process in which a threshold value is subtracted from coefficient data whose absolute value is larger than the threshold value. Here, coefficient data F (0, 0) of a direct current component are excluded from thresholding.
In feedback type buffering as mentioned above, the feedback control of the quantization step and the threshold value is performed by the buffer control circuit 58, so that when the buffer memory 57 is nearly saturated and about to overflow, the transmission rate of the input data to the buffer memory 57 is lowered, whereas when the buffer memory 57 is relatively empty, the rate of the input data to the buffer memory is increased. If the sensitivity to the feedback amount is increased excessively for the feedback control, oscillation takes place in the vicinity of the object value. Conversely, when the sensitivity is too low, a problem develops, in that, it takes a relatively long time for convergence which necessitates increasing the capacity of the buffer memory 57. As a result, a considerable amount of know-how is required to utilize the conventional buffering process effectively.
A further disadvantage with the conventional feedback type buffering device is that complicated circuits, such as the sorting circuit 55 and the thresholding circuit, are required.
Further, although the transmission data amount in the conventional buffering system can be lowered on the average over a long period so as to be less than a predetermined value, it is difficult to control the data amount correctly for a unit of one field or one frame of a television signal as utilized, for example, in a digital VTR.
The present applicant has proposed a highly efficient coding apparatus which encodes utilizing (ADRC adaptive dynamic range coding) coefficient data previously obtained through transform encoding and suppresses the data amount of an encoded output at a value equal to or less than a predetermined value (refer to Japanese Patent Application Sho 63-245227). Although this system solves disadvantages associated with the conventional feedback type buffering and increases the data compression rate, by combining an ADRC encoding apparatus, the circuit becomes more complicated with increased data errors.